1. Field of the Invention
The present invention relates to an automatic transmission used with a torque converter, and in particular to a planetary gear mechanism.
Known automatic transmissions, in general, include only a single planetary gear mechanism having one pinion (a Simpson Type transmission, for example), at least one dual planetary gear mechanism having plural pinions (a Ravigneaux Type transmission for example) and the type proposed in the Japanese Pat. No. 60-230809, etc. The present invention relates to the latter, particularly to a dual planetary gear mechanism.
2. Description of the Prior Art
As shown in FIG. 11, a dual planetary gear generally comprises a sun gear S', a ring gear R', a first planetary pinion P', a second planetary pinion Q' and a carrier CR' supporting the first and second planetary pinions P' and Q'. Usually, these planetary pinions P', Q' each consist of a single component, respectively. The planetary pinion P' and Q' mesh with each other. And P', Q' also mesh with the sun gear S' or the ring gear R'.
A dual planetary gear unit is often used in combination with a single planetary gear unit. For example, in the Ravigneaux Type transmission, sun gears are respectively incorporated in each single and dual planetary gear mechanism; a second pinion, and a common ring gear and carrier are used in the single and dual gear units. One of the sun gears, a common (the second) pinion and the ring gear make up a single planetary gear unit; another sun gear, the common pinion and the first pinion which is disposed between the "another sun gear" and the common pinion make up a dual planetary gear unit. And, a transmission proposed by the applicant of the present invention, ring gears are respectively incorporated in each single and dual planetary gear mechanism; the first pinion, and a common sun gear and carrier are used in the single and dual gear unit. One of the ring gears, a common (the first) pinion and a sun gear make up a single planetary gear unit; another ring gear, the common pinion and the second pinion which is disposed between the "another ring gear" and the common pinion make up a dual planetary gear unit.
Accordingly the first pinion (the common pinion) can be axially long. However, the second pinion cannot be axially long because of the interference of the sun gear or the ring gear of the single planetary gear unit. Due to the above, cog pressure generated between the second pinion and the sun gear (Ravigneaux Type) or the ring gear (proposed) is much higher than that generated between the common pinion and the ring gear (Ravigneaux Type) or the sun gear (proposed), and so much pressure may limit the total torque transmission.
To reduce the cog pressure, the planetary gear unit is made wider or the number of pairs of the pinions is increased (for example, 4 pairs, etc.). However, such results in the size of whole transmission becoming relatively large. Consequently, the above solution cannot meet a requirement of a compact transmission for a front-mounted engine front-wheel drive car.